Recently, a sharply increasing demand for and multimedia-systemization of mobile communication systems requires large-scale and high-speed communication systems. According to the increasing amount of information to be communicated, frequency bands to be used are being widened from microwave bands to millimeter-wave (milliwave) bands. In such milliwave bands, conventionally-known TE01.delta.-mode dielectric resonators composed of a cylindrical dielectric device can be used in a manner similar to the case of microwave bands. In this case, since the frequency of the TE01.delta.-mode dielectric resonator is defined according to the outer dimension of the cylindrical dielectric device, strict processing accuracy is required.
Also, suppose a dielectric filter is configured by arranging a plurality of the TE01.delta.-mode dielectric resonators to be apart from each other with a predetermined spacing in a metal housing. In such a case, coupling between an input/output means such as a metal loop and a dielectric resonator or between a dielectric resonator and a dielectric resonator is determined according to the distance therebetween. Therefore, the arrangement requires high positional accuracy.
Then, in Japanese Unexamined Patent Application Publication No. 7-62625, U.S. Pat. No. 5,764,116 the present applicant proposed a dielectric resonator and a dielectric filter that allow improved processing accuracy, solving the problems described above.
The dielectric filter according to the above patent application is shown FIG. 12. FIG. 12 is an exploded perspective view of the dielectric filter according to the above patent application.
As shown in FIG. 12, a dielectric filter 101 is constituted of a dielectric substrate 102 and conductor plates 104a and 104b.
The dielectric substrate 102 has a constant relative dielectric constant, on which electric conductors 102a and 102b having circular openings on their two main faces are formed so that that the openings on the two main faces oppose each other.
An input coplanar line 105a and an output coplanar line 105b are formed so as to be in proximity to two ends of the five openings on one of the main faces of the dielectric substrate 102 (the upper side in FIG. 12).
The dielectric plates 104a and 104b are immobilized such that they are spaced apart by a predetermined distance from the dielectric substrate 102 and so that they sandwich the dielectric substrate 102. The input coplanar line 105a and the output coplanar line 105B are projected from the dielectric plates 104a and 104b. Cutouts are arranged on the conductor plate 104a so that the input coplanar line 105a and the output coplanar line 105b are not connected. The conductor plate 104a and the electric conductor 102a of the dielectric substrate 102 are electrically connected, and the conductor plate 104b and the electric conductor 102b of the dielectric substrate 102 are electrically connected.
In the configuration as described above, electromagnetic-field energy is confined in the dielectric substrate 102 in the vicinity sandwiched by the openings opposing the electric conductors 102a and 102b, and five resonating sections are formed. Further adjacent resonating sections are coupled; thus, a dielectric filter having resonating sections in five steps is configured.
As described above, the resonating section can be defined according to the size of the opening of an electrode. This enables a processing means such as etching to be used in production and allows production of a dielectric resonator, a dielectric filter, and the like that have precisely reproduced dimensional accuracy of the resonating section.
In the dielectric filter 101 as described above, confinement of electromagnetic-field energy is high in the resonating sections formed by the dielectric substrate 102 sandwiched by the openings on the opposing electric conductors 102a and 102b. Therefore, when an input/output terminal means is formed of the coplanar lines 105a and 105b, coupling is weak between the resonating sections and the input/output terminal means. Therefore the distance between the openings of the electrodes 102a and 102b and the input coplanar lines 104a and 104b is shortened as much as possible so as to strengthen coupling between the resonating sections and the input/output terminal means.
Also, in the dielectric filter 101 as described above, since confinement of electromagnetic-field energy is high in the resonating sections, coupling is weak between the adjacent resonating sections. Therefore the distance between the openings is shortened as much as possible so as to strengthen coupling between the resonating sections.
In addition, a conventionally as an apparatus using a dielectric resonator, namely a voltage-controlled oscillator, is shown in FIG. 13.
As shown in FIG. 13, a voltage-controlled oscillator 111 uses a cylindrical TE01.delta.-mode dielectric resonator 112.
The TE01.delta.-mode dielectric resonator 112 is mounted on a wiring substrate 113 via a supporting base 112a. On a lower face of the wiring substrate 113, ground electrodes, not shown, are formed. The wiring substrate 113 is housed within an upper metal housing 130 and a lower metal housing 131.
On the wiring substrate 113, a microstrip line 114 composing a primary line and a microstrip line 115 composing a secondary line are formed so as to overlap each other as viewed downward from points over the TE01.delta.-mode dielectric resonator 112 and FIG. 13.
The microstrip line 114 is arranged such that one end thereof is connected to a ground electrode 117 via a chip resistor 116, and the other end thereof is connected to a gate of a field-effect transistor 118.
A resonating section is formed by electromagnetic-field coupling between the primary line composing the primary line and the TE01.delta.-mode dielectric resonator 112.
The microstrip line 115 is arranged such that one end thereof is connected to the ground electrode 117 via a varactor diode 119, and the other end thereof is an open end.
A variable oscillation frequency circuit is comprised of the microstrip line 115 composing the primary line and the varactor diode 119.
The field-effect transistor 118 is arranged such that a drain thereof is connected to an input terminal 122 via a microstrip line 121, and a source thereof is connected to one end of a microstrip line 123.
The microstrip line 121 is connected to a matching stub 124 at a point of connection with the drain of the field-effect transistor 118.
The other end of the microstrip line 123 is connected to the ground electrode 117 via a chip resistor 125. The microstrip 123 is formed so as to be parallel from a point with a microstrip line 126 with a constant distance so as to be electrically coupled.
The microstrip line 126 is connected to an output terminal electrode 128 via a chip resistor 127.
The matching stub 124 is connected to the input terminal electrode 122 in parallel with the microstrip line 121.
A chip capacitor 129 is connected to the output terminal electrode 128 in parallel with the chip resistor 127.
In a configuration such as that described above, the varactor diode 119 serves as a variable capacitor according to application voltages to vary resonance frequency, by which oscillation frequency varies.
As described above, in the dielectric filter 101 shown in FIG. 12, the distance between the openings of the electric conductors 102a and 102b and the input and output coplanar lines 105a and 105b is shortened as much as possible so as to strengthen coupling between the resonating sections and the input/output terminal means.
However, because of a limit to shortening of the distance between the openings of the electric conductors 102a and 102b and the input and output coplanar lines 105a and 105b, the coupling strength cannot be further increased.
Also, the length of the dielectric substrate 102 is increased in the direction of the resonating-section arrangement by formation of the input/output coplanar lines 105a and 105b, increasing the overall length of the dielectric filter 101. Therefore, the space for the input/output terminal means such as the input/output coplanar lines 105a and 105b is an obstacle to reducing the overall size of the dielectric filter 101.
Also, as shown in FIG. 12, when the dielectric filter having five-step resonating sections is so configured, five openings must be formed on the electric conductors 102a and 102b on the two main faces of the dielectric substrate 102. Accordingly, the overall size of the dielectric substrate 102 is increased, and as a result, the overall size of the dielectric filter 101 is increased. Therefore, the overall size of the dielectric filter is increased in proportion to the increase in the number of the openings on the electric conductors formed on the two main faces of the dielectric substrate, that is, the number of steps in the resonating sections.
Also, characteristics of the individual resonating section in the dielectric filter, such as frequency characteristics, are adjusted by eliminating electric conductors in the vicinity of the openings on the electrodes forming the resonating sections. However, since this changes the shape of the openings, electromagnetic fields are caused to diverge, and unnecessary spurious components are occasionally produced.
Also, when coupling between the individual resonating sections in the dielectric filter must be strengthened, the distance between the openings in the electric conductor of the electric conductor is shortened. That is, a different dielectric substrate having a smaller distance between openings of the electric conductor is used, and a different dielectric substrate must be prepared. This takes time and incurs costs.
Also, to adjust characteristics of the dielectric filter, for example, to indirectly couple resonating sections separated from each other, a different capacitor, a coil, and the like, and circuit elements such as lead lines formed on the dielectric substrate 102 are arranged on the dielectric substrate 102. Also, to arrange these circuit elements on the dielectric substrate 102, lead lines for arranging them are also formed on the same substrate. When such lead lines are formed around the resonating sections, however, the dimensions of the substrate used must be larger, the size of the dielectric device is increased, and the overall size of the dielectric filter is also increased.
In addition, in the voltage-controlled oscillator 111, electromagnetic fields of the TE01.delta.-mode dielectric resonator 112 are widely dispersed around the TE01.delta.-mode dielectric resonator 112. Therefore, a problem arises in that the electromagnetic fields couple to the microstrip lines 121 and 123 and the like, instead of the microstrip line 114 and the microstrip line 115. When such unnecessary coupling occurs, the oscillation frequency in the voltage-controlled oscillator 111 may be unstable. Conventionally, to minimize defects due to such unnecessary coupling, wiring was designed so that the microstrip lines 121 and 123 which is not desired to be coupled to the TE01.delta.-mode dielectric resonator 112 are separated as far as possible from the TE01.delta.-mode dielectric resonator 112.
However, separation of microstrip lines other than the primary line and secondary line requires the wiring substrate 113 to be enlarged proportionally to the separation, resulting in enlargement of the overall size of the voltage-controlled oscillator 111.
Also, since wiring is designed under the condition that the microstrip lines 121 and 123 which are not desired to be coupled to the TE01.delta.-mode dielectric resonator 112 are separated as far as possible from the TE01.delta.-mode dielectric resonator 112, less flexibility remains in the wiring design.
Also, the TE01.delta.-mode dielectric resonator 112 is arranged on the wiring substrate 113, and the wiring substrate 113 is covered by the upper metal housing 130 so as to confine electromagnetic fields in the TE01.delta.-mode dielectric resonator 112. In this case, the height of the upper metal housing 130 must be made larger than that of the TE01.delta.-mode dielectric resonator 112. This also increases the height of the voltage-controlled oscillator 111.